Floating units, e.g. reconverted ships, are known to be used for off-shore oil extraction, are commonly known as floating production units (FPUs), and comprise so-called FSO (floating storage and offloading) units, and FPSO (floating production, storage and offloading) units.
A floating production unit is normally anchored permanently to the bottom by a number of mooring lines extending from respective mooring stations on the unit.
A mooring line normally comprises a first chain portion fixed to a mooring station by a fastening device; a cable portion (e.g. made of synthetic material); and a second chain portion terminating with an anchor.
Spread mooring is one of various known mooring configurations, and comprises two groups of mooring lines at respective ends of the unit (e.g. at the bow and stern, in the case of a reconverted ship).
Each mooring line must obviously be tensioned. In the case of spread mooring, this is done using a number of independent winches for respective mooring lines, or two winches, each located, and catering to a group of mooring lines, at a respective end of the unit. Both solutions employ hydraulic chain winches known as windlasses, i.e. vertical-axis winches with a chain-winding indented drum. As is known, winches of this sort are extremely bulky, complicated, expensive, and difficult to procure, so installing a number of them involves considerable cost and often fairly long delays awaiting delivery.
Moreover, unless each winch is equipped with its own electrohydraulic system, employing a number of hydraulic winches located at various points on the unit calls for a complex piping system to connect the winches to a common electrohydraulic system.
On known floating production units, the mooring line tensioning winches cannot be used for other purposes, in particular for hoisting any production lines the unit may be equipped with. As is known, oil production lines are normally gathered by a dedicated structure (“riser balcony”) which, for hoisting and handling production lines in general, is equipped with special winches separate from and independent of the mooring line tensioning winches.
The known solutions described briefly above, as well as other similar solutions, therefore have a number of drawbacks.